New Chernobyl Disaster Findings May Help Estimate Cancer Risk from Low-Dose Exposures from CT Scans

A 20-year study has been conducted that tracked 110,645 Ukrainian workers who helped clean up after the 1986 Chernobyl nuclear power plant accident. The new findings revealed that the workers share a considerable increased chance of developing leukemia. The new findings may help scientists better determine cancer risks associated with low doses of radiation from diagnostic imaging procedures that utilize radiation such as computed tomography (CT) scans and other sources.

In the November 8, 2012, issue of the journal Environmental Health Perspectives, an international team led by scientists from the University of California, San Francisco (UCSF; USA) and the Chernobyl Research Unit at the Radiation Epidemiology Branch of the National Cancer Institute describes the increased risks of leukemia among these workers between 1986 and 2006. The risk included a greater-than-expected number of cases of chronic lymphocytic leukemia, which many specialists did not consider to be linked with radiation exposure in the past.

The new study is the longest and largest study to date involving Chernobyl cleanup workers who worked at or near the nuclear complex in the aftermath of the accident. Overall, there were 137 cases of leukemia among the workers over the 20-year span of the study, and 16% of those cancers were attributable to the Chernobyl radiation exposure, the investigators found.

The findings offer clues into on the thorny issue of estimating cancer risk from low doses of radiation --a matter of importance to nuclear workers, miners, and anyone who is chronically exposed to low levels of radiation at work or patients who receive substantial radiation doses when undergoing medical diagnostic tests. “Low doses of radiation are important,” said the lead researcher Lydia Zablotska, MD, PhD, an associate professor of epidemiology and biostatistics at UCSF. “We want to raise awareness of that.”

The nearly 111,000 Ukrainian workers in the study were among the more than 500,000 former Soviet [USSR] citizens who worked directly on the nuclear complex site in the aftermath of Chernobyl disaster, which was the worst nuclear accident of the 20th century followed by the 2011 Fukushima disaster in Japan. On April 26, 1986, a planned test of a backup system for running cooling pumps went haywire. A combination of unsafe reactor design and human error and led to the cascading generation of heat in Chernobyl’s reactor no. 4, which rapidly caused two massive explosions, ruptured the reactor, crushed the building, drizzled radioactive debris around the compound, and spread fallout through the atmosphere over the Soviet Union and Europe.

Many of the Ukrainian workers were exposed to high levels of radiation because they were part of the heroic groups that helped clean up contaminated wreckage from the immediate area--much of which was highly radioactive. Some of them, in fact, reached lifetime limits of radiation exposure within a few hours. Even though an elevated radiation-related risk of leukemia was not unexpected, given the degree of exposure among many of these workers, what did surprise Dr. Zablotska and her colleagues was the increased risk of chronic lymphocytic leukemia (CLL), which was similar in size to the risk estimated for non-CLL leukemia.

Clinicians have known for a long time that X-ray source ionizing radiation or generated by the decay of radioactive elements can cause leukemia, because it can penetrate the body, expose bone marrow to the radiation, and impair DNA. However, whereas scientists have understood specifics about this basic process, how much leukemia risk is associated with moderate or low doses of radiation have been difficult to assess.

For many years, the best estimates came from long-term studies involving survivors of the 1945 atomic bomb explosions over Nagasaki and Hiroshima, Japan, during World War II. People in the close vicinity of the blasts were exposed to various levels of radiation, and in the years afterward, their health was monitored and the increase in cancer tracked.

From those cancer risk evaluations, scientists estimated risks from lower doses by extrapolating the data down. But there have always been snags with this approach, according to Dr. Zablotska. Atomic bomb survivors were immersed in neutron or gamma rays, where when patients undergo CT scanning in the United States, they are exposed to X-rays, a different sort of radiation. Moreover, deducing risks for Japanese population to Western population is additionally muddled by disparities in lifestyle, genetics, and diet between the two.

The new study helps to close this discrepancy because the doses received by the Ukrainian cleanup workers fell between the high level received by the Japanese atomic bomb victims and the lower levels received by people who undergo extensive medical scans. It also challenges the hypothesis that chronic lymphocytic leukemia is not linked to radiation exposure--something that earlier research of atomic bomb survivors had appeared to backup.

The genetic composition of the Japanese population may have hidden any increased risk, according to Dr. Zablotska, because they are much less likely to develop this type of cancer. Chronic lymphocytic leukemia accounts for only 3% of all cases of leukemia in Japan—in contrast to approximately one-third of all leukemia cases in the United States and 40% of all cases of leukemia in Ukraine.

Other authors on this study are associated with the National Research Center for Radiation Medicine (Kyiv, Ukraine), the US National Cancer Institute (Bethesda, MD, USA), Robert Wood Johnson Medical School (Camden, NJ, USA), Columbia University (New York, NY, USA), and Burnasyan Federal Medical Biophysical Center (Moscow, Russia).

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